During the past year, we continued to study a group of 13 monoclonal antibodies prepared by immunizing rats with mouse brain. Nine have been partially analyzed. Four react with multipotential stem cells (CFUs). Only one of the monoclonal antibodies that kill CFUs also reacts with prothymocytes. Another of these reagents reacts with all B cells. Two other antibodies have been extensively characterized; these react with, and are cytotoxic for, prothymocytes (AE3.38 and BG2.17). Another of these monoclonal antibodies, AE3.d3, is mitogenic for mouse lymphocytes. B lymphocytes are the most susceptible to this proliferative stimulus, and they are induced to differentiate into plaque-forming cells. The mitogenic response is not genetically restricted by H-2 type; and B-cell deficient mouse strains such as C3H/HeJ and CBA/N, as well as T-cell deficient strains such as BALB/c nude mice, are capable of responding to the stimulus of AE3.d3 (AE3.d3-positive cells). The highest percentage is found in the spleen (approximately 28%) and lymph node (18%), whereas only 14% of the bone marrow and 5% of the cells of the thymus are brightly stained with AE3.d3. Much of the work this past year has been characterizing the photoactive reagent diazidobenzoyl cystine, an aryl azide containing both a radioactive isotope label and a disulfide capable of interacting with proteins. This reagent has been attached to a variety of proteins, and groups of mice have been immunized with each conjugate. Monoclonal antibodies were produced and used to generate ascites in mice. The resulting antibody was reacted with the photoaffinity-labeled antigen. T cells from the mice immunized with this antigen were also placed in culture, and lines that seem to have reactivity with diABC are being maintained. (LB)